It is a general feature of processes of the kind in question that changes in particulate properties (chemical or physical), and/or changes in process conditions which lead to adhesion or sticking of particles either to each other or to process equipment, can lead to a decrease in process efficiency (for example through decreased mixing and mass transfer in fluidised beds or kilns) or process throughput (for example through interruptions to flow of the particulate solids within or between reactor vessels). Also, in some processes there can be a gradual agglomeration or sintering of particulate solids that can interrupt the process requiring corrective action, and eventually decreases the process capability to the extent that it has to be shut down for a period to allow physical removal of the accretions formed.
Examples of DRI processes in which particulate stickiness, adhesion and flow properties are key factors, include the FIOR™, FINMET™, FINEX™ and SL/RN kiln processes.
It is known that both process conditions and properties of the particulate solids influence the degree of stickiness or adhesion. The readiness or otherwise with which particulate solids in a process tend to stick or adhere to each other or process equipment is sometimes expressed qualitatively in terms of the “stickiness” of the solids for the particular conditions in question, but to date there has been no method for providing a reliable, quantitative measure of this quality. Particulate solids, eg iron-containing fines, appear in some conditions to have a particular disposition to be sticky, which can, for example, lead to “bogging” or defluidisation in fluidised beds, poor solids flow between process vessels, and formation of accretions in fluidised beds and rotary kilns.
To date, there has been no reliable measure to quantify this “stickiness” and it has proven difficult to predict the adhesion, stickiness or agglomeration behaviour of particulate solids impacting the process performance (efficiency, throughput and availability). This is particularly the case for higher temperature processes involving mineral particles, where complex phases exist that influence particulate behaviour, and to manage this stickiness/adhesion behaviour and/or accretion formation has involved somewhat imprecise empirical methods. These have been less than satisfactory given the wide number of parameters that appear to be involved.
Several investigations have attempted to simulate the real behaviour of particulate solids during direct reduction of iron ores using laboratory scale fluidised bed reactors in conjunction with optical and/or scanning electron microscopy, eg, Gransden and Sheasby (1974), Astier and Roux (1975), Hyashi and Iguchi (1992), Janssen (1994), Gudenau et al (1997). These test methods rely on being able to simulate the gas, temperature, chemical reaction rates and fluid dynamic conditions in a process. As this is rarely possible, the tests give only a qualitative comparison of sticking behaviour. Mikami et al (1996) followed a similar procedure to investigate sticking during the manufacture of iron powder for powder metallurgy. The apparatus for these tests is complex and large samples are typically required.
Other workers have utilised standard ash fusion and compression tests to estimate the agglomeration tendency of ash in fluidised bed combustors, eg Conn (1994), and Skrifvars et al (1999). However, these tests do not have a flow component and therefore tend to overestimate the agglomeration temperature.
Conn (1994) proposed a drained angle of repose test which measured the flow properties of approximately 1.4 kg of sample under appropriate temperature conditions. This test has the advantage of measuring flow properties, but requires large samples and it may be difficult to provide a controlled gas atmosphere.